Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of aftertreatment systems on engines to reduce emissions is increasing. Typical aftertreatment systems can include an oxidation catalyst, a particulate matter (PM) filter, a selective catalyst reduction (SCR) system, and an ammonia oxidation (AMOX) catalyst. The oxidation catalyst is used for reducing liquid hydrocarbons (e.g., soluble organic fraction (SOF)) in the exhaust gas stream. The PM filter is used to filter particulate matter from the exhaust gas stream. The SCR system is used to reduce NOx in the exhaust gas stream. The AMOX catalyst is used to reduce unused ammonia remaining in the exhaust gas stream after treatment by the SCR system.
The performance of aftertreatment systems are dependent upon the physical and chemical properties of the exhaust gas delivered from an internal combustion engine. The physical and chemical properties of exhaust gas are at least partially based on the engine control system's control of combustion, air-handling, and fuel. In typical systems, the engine control system, and thus the properties of exhaust delivered by an engine, does not consider the performance of the aftertreatment system in controlling the properties of the exhaust delivered by the engine. Rather, exhaust properties are controlled by the engine's control system based upon considerations exclusive of aftertreatment system performance. Accordingly, the components of conventional aftertreatment systems are configured to react to the physical and chemical properties of the exhaust gas according to desired exhaust emission targets. Often, the reactionary nature of conventional aftertreatment systems reduces the efficiency and performance of the aftertreatment system because of the transitory nature of combustion engines and inherent delays associated with broad post-combustion adjustment of the properties of the aftertreatment systems.
For example, SCR systems generate ammonia to reduce the NOx. When just the proper amount of ammonia is available at the SCR catalyst under the proper conditions, the ammonia is utilized to reduce NOx. However, if the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip out the exhaust pipe. Further, conventional SCR systems that utilize injected urea to produce ammonia must account for potential delays in the vaporization and hydrolysis of urea to ammonia. Additionally, SCR systems that utilize urea dosing to generate ammonia depend upon the real-time delivery of urea to the SCR catalyst as engine NOx emissions emerge. Urea dosers have relatively slow physical dynamics compared to other chemical injectors such as hydrocarbon injectors. Therefore, post-combustion adjustments in urea dosing can be delayed due to the urea doser dynamics of conventional SCR controls systems.
The inherent reactionary delays of conventional exhaust aftertreatment control systems are accentuated by transient and unpredicted exhaust properties associated with exhaust aftertreatment systems that do not control the exhaust properties delivered by the engine. Accordingly, a need exists for an exhaust aftertreatment control system that manipulates the properties of engine exhaust at least partially through the engine's control system to increase the efficiency and performance of the exhaust aftertreatment system.